Kazunari Matsuda

Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping

We demonstrate the tunability of the photoluminescence (PL) properties of monolayer (1L)-MoS2 via chemical doping. The PL intensity of 1L-MoS2 was drastically enhanced by the adsorption of p-type dopants with high electron affinity but reduced by the adsorption of n-type dopants. This PL modulation results from switching between exciton PL and trion PL depending on carrier density in 1L-MoS2. Achievement of the extraction and injection of carriers in 1L-MoS2 by this solution-based chemical doping method enables convenient control of optical and electrical properties of atomically thin MoS2.

Near-field optical fiber probe optimized for illumination–collection hybrid mode operation

The structure of a near-field probe was optimized for illumination-collection hybrid mode (I–C mode) operation. We developed a highly sensitive probe with a sharp-edged aperture and a double-tapered structure fabricated by a chemical etching technique. Through measurement of the photoluminescence (PL, λ=1 μm) image of single quantum dots, the performance of many versions of the probe with different aperture diameters, ranging from 80 to 500 nm, was systematically evaluated. In addition to the throughput and spatial resolution, the absolute value of the PL collection efficiency of the probe in (I–C mode) was also estimated for the first time. A collection efficiency several times greater than that of an objective lens with a numerical aperture of 0.8 was achieved with high spatial resolution in the 130–200 nm (λ/8–λ/5) range.

Observation of Charged Excitons in Hole-Doped Carbon Nanotubes Using Photoluminescence and Absorption Spectroscopy

We report the first observation of trions (charged excitons), three-particle bound states consisting of one electron and two holes, in hole-doped carbon nanotubes at room temperature. When p-type dopants are added to carbon nanotube solutions, the photoluminescence and absorption peaks of the trions appear far below the E11 bright exciton peak, regardless of the dopant species. The unexpectedly large energy separation between the bright excitons and the trions is attributed to the strong electron-hole exchange interaction in carbon nanotubes.

Photoluminescence enhancement and quenching of single CdSe∕ZnS nanocrystals on metal surfaces dominated by plasmon resonant energy transfer

We studied the mechanism of the photoluminescence (PL) enhancement and quenching of single CdSe∕ZnS nanocrystals on rough Au surfaces. Single nanocrystal spectroscopy revealed that the PL enhancement depends strongly on the excitation wavelength and liner-polarization angle due to the localized plasmon excitation and is also sensitive to the nanocrystal size. The polarization- and size-dependent PL enhancement and quenching are determined by the balance between the resonant energy transfer from the nanocrystal to the Au surface and the electric field enhancement.

Considerably improved photovoltaic performance of carbon nanotube-based solar cells using metal oxide layers

Carbon nanotube-based solar cells have been extensively studied from the perspective of potential application. Here we demonstrated a significant improvement of the carbon nanotube solar cells by the use of metal oxide layers for efficient carrier transport. The metal oxides also serve as an antireflection layer and an efficient carrier dopant, leading to a reduction in the loss of the incident solar light and an increase in the photocurrent, respectively. As a consequence, the photovoltaic performance of both p-single-walled carbon nanotube (SWNT)/n-Si and n-SWNT/p-Si heterojunction solar cells using MoOx and ZnO layers is improved, resulting in very high photovoltaic conversion efficiencies of 17.0 and 4.0%, respectively. These findings regarding the use of metal oxides as multifunctional layers suggest that metal oxide layers could improve the performance of various electronic devices based on carbon nanotubes.

Simultaneous Discrimination of Handedness and Diameter of Single-Walled Carbon Nanotubes (SWNTs) with Chiral Diporphyrin Nanotweezers Leading to Enrichment of a Single Enantiomer of (6,5)-SWNTs

Separation of single-walled carbon nanotubes (SWNTs) according to their handedness has been attracting growing interest. Our methodology to separate the enantiomers of SWNTs is based on molecular recognition with chiral diporphyrin nanotweezers. Herein, we report novel nanotweezers 1 consisting of two chiral porphyrins and phenanthrene in between. These nanotweezers 1 are rationally designed to discriminate diameter and handedness simultaneously by taking into account the relationship between the (n, m) selectivity and the structures of previously reported chiral nanotweezers. Owing to the relatively narrow cleft made by two porphyrins, the nanotweezers 1 showed high selectivity toward (6,5)-SWNTs possessing the smallest diameter among the major components of CoMoCAT-SWNTs. In addition, the chiral diporphyrin 1 discriminated the left- and right-handed structures of (6,5)-SWNTs, providing high enantiomeric excess (67% ee on the basis of the (6,5)-SWNTs with high optical purity recently reported by Weisman). In conclusion, only the single stereoisomer of (6,5)-SWNTs was highly enriched through the extraction of CoMoCAT-SWNTs with phenanthrene-bridged chiral diporphyrin nanotweezers 1.

Nonlinear photoluminescence in atomically thin layered WSe2 arising from diffusion-assisted exciton-exciton annihilation

We studied multiexciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density and long-distance exciton diffusion, reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation (EEA) model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient EEA process determined the unusual multiexciton dynamics in atomically thin layered transition metal dichalcogenides.

Room-temperature photoluminescence spectroscopy of self-assembled In0.5Ga0.5As single quantum dots by using highly sensitive near-field scanning optical microscope

We have studied the optical properties of self-assembled In0.5Ga0.5As single quantum dots (QDs) at room temperature with a near-field scanning optical microscope. Successful detection of a weak photoluminescence (PL) signal from a single QD at room temperature could be achieved by using a double-tapered fiber probe having the advantages of both high collection efficiency and high spatial resolution. Through the precise examination of PL spectra of many QDs, including broadening and saturation behaviors, the homogeneous linewidth of the ground state emission is evaluated as from 9.8 to 14.5 meV.

Carrier multiplication in carbon nanotubes studied by femtosecond pump-probe spectroscopy

Carbon nanotubes are one of the excellent materials for studying the many-body effects of excitons because of their unique band structures and large exciton binding energies. We studied exciton population dynamics in single-walled carbon nanotubes using pump-probe transient absorption measurements. The temporal profiles of the transient absorption signals depend on the excitation intensity and excitation photon energy. We observe carrier multiplication in carbon nanotubes at room temperature, when the excitation photon energy exceeds the third subband exciton energy.

Exploring the Origin of Blue and Ultraviolet Fluorescence in Graphene Oxide

We studied the fluorescence (FL) properties of highly exfoliated graphene oxide (GO) in aqueous solution using continuous-wave and time-resolved FL spectroscopy. The FL spectra of highly exfoliated GO showed two distinct peaks at ∼440 (blue) and ∼300 nm [ultraviolet (UV)]. The FL of GO in the UV region at ∼300 nm was observed for the first time. The average FL lifetimes of the emission peaks at ∼440 and ∼300 nm are 8-13 and 6-8 ns, respectively. The experimentally observed peak wavelengths of pH-dependent FL, FL excitation spectra, and the FL lifetimes are nearly coincident with those of aromatic compounds bound with oxygen functional groups, which suggests that the FL comes from sp(2) fragments consisting of small numbers of aromatic rings with oxygen functional groups acting as FL centers in the GO.